So mail tips over immediately on launch?
Does mail work from another newly-created login? If so, then there's something wonky with your own login environment.
If mail (also) does not work from a different login user, then (re)download the OS X 10.6.8 combo update from Apple - the combo kit, and not the delta kit - and reinstall it. That'll reload all of the changes that 10.6.8 brought along.